1. Field of the Invention
The invention is related to the field of optical devices, and in particular, to meso-optic optical devices.
2. Statement of the Problem
A useful characteristic of an optic or optical element is the ability of the optic to concentrate or focus electromagnetic radiation, or conversely, to direct the emission of radiation from an area along a predetermined ray path or axis, i.e. optical axis. It is equally useful to focus radiation into the form of a line coincident to an optical axis. An axicon possessing a refracting conical surface as shown in FIG. 1 provides a means to focus energy into a line-of-focus substantially coincident to the optical axis. An axicon with a refracting conical surface cannot, however, focus radiation substantially perpendicular to the optical axis.
A reflective forty-five degree annular axicon 1 of the prior art possessing a single reflective conical optical surface, as shown in FIG. 2, focuses radiation 5 substantially perpendicular to an focal axis separated from the axis of revolution by the incident angle β; i.e. the angular separation of the focal axis to the axis of revolution is related to the incident angle and forms a conical volume of focus with apex coincident with the apparent apex of the conical optical surface. Therefore, radiation does not fall perpendicular to the optical axis 3 or upon the axis of revolution 2. Only in special conditions where the axis of revolution and the optical axis are coincident does radiation fall perpendicular to the optical axis.
A reflective axicon of the prior art possessing more than one reflective conical optical surface as shown in FIG. 3, one form of a w-axicon, can act as a beam expander/reducer or shaper, modifying a propagating beam of radiation from a circular cross-section to a beam with annular cross-section or visa-versa, or can act much like a simple refractive axicon focusing radiation to a line along the optical axis. A reflection axicon of the prior art with two or more conic surfaces includes an ability to focus of radiation of along the optical axis. However, this prior art optic cannot focus radiation substantially perpendicular to the optical axis and is not able to direct radiation emitted radially divergently from the optical axis, rendering this optical configuration unsuitable in applications wherein substantial radial divergence from the line-of-focus is a requirement.
A w-axicon of the prior art with two forty-five degree conical optical surfaces as shown in FIG. 4, acting as a retro-reflector, returns the reflected beam towards the source of the radiation along the optical axis after reflecting from both conical optical surfaces, but does not form a line-of-focus. Used as a beam expander/reducer or beam shaper, the w-axicon of FIG. 4 returns the reflected beam towards the source of the radiation along the optical axis, but does so by illuminating first one conical optical surface and next the other conical optical surface, changing the shape of the incident beam from a circular cross-section to an annular cross-section or visa-versa.
FIG. 5 is a section view of a plano-convex lens of the prior art showing the conventions used to describe the relationship between the radius-of-curvature and saggita of a surface-of-revolution as applied to conic surfaces.
A desirable attribute of a meso-optic is the ability to collimate radiation that is radially divergent from a line-of-focus or cylindrical surface, and to redirect radiation to impinge substantially perpendicularly upon a cylindrical surface to a direction substantially parallel to axis of the cylindrical surface, i.e. a cylindrical focal surface. A cylindrical focal surface is useful for the stimulation of laser materials, especially those which are in the form of a cylinder or rod, solar concentrators, and for the detection and measurement of electromagnetic emissions at right angle to a stimulus beam of energy; as in the case wherein fluorescence or nephelometric measurements are to be made substantially at a right angle from the stimulating radiation. In order to detect low level electromagnetic events, such as photons impingent upon a photodiode as result of scatter from a small concentrations of particles in a suspension, or from the emission of energy that results in fluorescence, the electrical signal produced by means of photovoltaic effect must be greater than the noise signal produced as result of quantum phenomenon of the detector material. The detection limit is the discernable event signal with respect to the noise signal, often regarded as an event signal twice the standard deviation of the mean noise signal. Since the inherent noise signal of the detector cannot be eliminated completely, and the amount of the electromagnetic radiation available for detection is often limited, it is advantageous to collect and concentrate as much of the electromagnetic event onto the detection means as possible in order to maximize the signal-to-noise ratio. Often, radiation emission is radial to the axis of a cylindrical emitting surface or cylindrical volume. Conventional optics, as that of a cylindrical lens which focuses light to a line, rectangle, or semi-cylindrical field, are limited in the subtended angle about the long axis of the cylindrical surface to which it can operate without excessive optical aberrations. In other words, a cylindrical lens can image only that portion of a cylindrical surface that is facing the lens. Multiple cylindrical lenses are needed in order to achieve three hundred and sixty degree coverage of a cylindrical focal plane. In addition, if the emitted energy from a cylindrical surface is to be directed along the axis of the cylindrical surface, additional optical elements in the form of mirrors are needed. Directing the emitted energy from a cylindrical surface along a single optical axis is advantageous, and as result conventional optics such as mirrors, prisms, and lenses can be used. A reflective axicon of the prior art as shown in FIG. 2 is able to redirect radiation that is emitted radially divergently from a cylindrical surface to a direction substantially parallel to axis of the cylindrical surface only in special condition where the axis of the cylinder and axis of revolution are coincident. An axicon of the prior art utilizing a single conic surface is limited as to the ability to form a line-of-focus close to the optical axis when the incident rays are not parallel to the optical axis. Indeed, an axicon of the prior art that attempts to focus radiation that is substantially perpendicular to the optical axis will suffer from excessive off-axis aberrations; i.e., a line-of-focus that is formed non-coincident to the optical axis is substantially distorted and out of focus.